A known seat apparatus is disclosed in JP2004155341A. This seat apparatus includes a frame member, a cushion body provided on the frame member, and a seat cushion, a seating surface of which is formed by the cushion body. The seat apparatus further includes a gear member, a driving mechanism and an arm. The gear member is rotatably supported at the frame member. The driving mechanism is supported at the frame member and, in order to rotate the gear member, connected to the gear member. The arm is swingably supported at the frame member, connected to a front-end portion of the cushion body and, through a link mechanism, connected to the gear member. As a result of the rotation of the gear member by means of an operation of the driving mechanism, the seat apparatus swings the arm thorough the link mechanism and, by operating the front-end portion of the cushion body upwards and downwards relative to the frame member, the seat apparatus varies a length of the seating surface.
The link mechanism includes a drive link and a driven link. A first end of the drive link is rotatably supported at the gear member through an elongated hole extended in a width direction of the drive link. A first end of the driven link is fixed to the arm, and a second end of which is supported at a second end of the drive link and is rotatable around a shaft extending in a width direction of the driven link. With the configuration of the aforementioned seat apparatus, in order to vary the length of the seating surface of the seat cushion, the arm is swung by operating the drive link and driven link in a longitudinal direction of the seat by means of rotation of the gear member. On such occasions, because of rotation of the driven link relative to the drive link, the driven link is made to follow a swing of the arm, and, because of a movement of the drive link relative to the gear member along the elongated hole, a locus error between a rotation locus of the gear member and a movement locus of the drive link is absorbed.
However, with the configuration of the aforementioned seat apparatus, in a condition where the length of the seating surface of the seat cushion is varied, in other words, in a condition where, in order to swing the arm, by means of the rotation of the gear member, the driven link and the drive link are moved in a longitudinal direction of the seat, a relative position between a fixed point of the first end of the driven link relative to the arm and a bearing point of the first end of the drive link relative to the gear member slips in a width direction, and distortions in the drive link and the driven link may on occasions be generated. Although such distortions are absorbed, to some degree, by means of looseness in terms of structural measurements at the fixed point of the first end of the driven link relative to the arm, the distortions and the looseness tend to interrupt a smooth swing of the arm. Moreover, the more that the assembling accuracy of the driven link at the fixed point of the first end of the driven link relative to the arm is enhanced, the greater the degree to which the swing of the arm itself is interrupted.
A need thus exists for a seating apparatus, which secures a reliable and smooth swing of an arm.